Integrated circuit with dynamic threshold voltage

ABSTRACT

An integrated circuit and method for making it are described. The integrated circuit includes a first insulating layer formed on a substrate and a body strap of a first conductivity type that is formed on the first insulating layer. A second insulating layer is formed on the first insulating layer adjacent to the body strap and a film is formed on the second insulating layer. The integrated circuit also includes a gate electrode formed on the film. A plurality of doped regions of a second conductivity type are formed within the film that extend from the surface of the film to the surface of the second insulating layer. The doped regions have junctions that are each spaced from the body strap by at least about 500 angstroms.

FIELD OF THE INVENTION

The present invention relates to integrated circuits and a method formaking them.

BACKGROUND OF THE INVENTION

As transistor gate lengths continue to shrink, threshold voltages(V_(t)) continue to decrease. Below 1.0 volts, it may become difficultto balance adequate current drive with acceptable leakage current. Oneway to address this problem is to design a device that has a dynamicthreshold voltage, i.e., a device that has a lower V_(t) when the deviceis turned on than when the device is turned off. By varying thethreshold voltage in this way, even a low voltage device may provide ahigh drive current while continuing to maintain low subthresholdleakage.

Devices having a dynamic threshold voltage may be made by tying a bodystrap formed below the transistor's channel to the transistor's gateelectrode, as described by Hu, et al., “A Dynamic Threshold VoltageMOSFET (DTMOS) for Ultra-Low Voltage Operation,” Int'l Electron DevicesMeeting Technical Digest, 1994, pp. 809-812; and Hu et al., “ChannelProfile Optimization and Device Design for Low-Power High-PerformanceDynamic-Threshold MOSFET,” Int'l Electron Devices Meeting TechnicalDigest, 1996, pp. 113-116.

Such devices may use silicon-on-insulator (“SOI”) technology toelectrically isolate the body strap from the junctions and adjacenttransistors. FIG. 1 illustrates such a device, which includes relativelythick silicon film 104 formed on oxide 103. As shown, body strap 102 isseparated from junctions 181 and 111, which correspond to the lowerboundaries for source 106 and drain 107, respectively. Because of thatseparation, the capacitance that may result, when the device is turnedon, can slow down the rate at which the body potential increases inresponse to an increase in the voltage applied to gate electrode 105.

One way to address that problem is to use a relatively thin siliconlayer instead of a relatively thick layer. FIG. 2 illustrates such adevice, which includes relatively thin silicon film 204 formed on oxide203. When a thin film is used, however, misalignment between body strap202 and gate electrode 205 can cause a short circuit between body strap202 and one of the junctions, e.g., junction 211 in the device shownhere.

There is a need for a device that does not include such short circuits,or produce unacceptable capacitance that slows down the rate of bodypotential increase when the device is turned on. There is also a needfor a method for making such a device.

SUMMARY OF THE INVENTION

An integrated circuit and method for making it are described. Theintegrated circuit includes a first insulating layer formed on asubstrate and a body strap of a first conductivity type that is formedon the first insulating layer. A second insulating layer is formed onthe first insulating layer adjacent to the body strap and a film isformed on the second insulating layer. A gate electrode is formed on thefilm and a plurality of doped regions of a second conductivity type areformed within the film that extend from the surface of the film to thesurface of the second insulating layer. The doped regions have junctionsthat are each spaced from the body strap by at least about 500angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a illustration of a cross-section of an SOI device thatincludes a relatively thick silicon film and a body strap used toprovide a dynamic V_(t).

FIG. 2 is an illustration of a cross-section of an SOI device thatincludes a relatively thin silicon film and a body strap used to providea dynamic V_(t).

FIG. 3 is an illustration of a cross-section of a section of anembodiment of the integrated circuit of the present invention.

FIG. 4 is an illustration of a cross-section of a section of a secondembodiment of the integrated circuit of the present invention.

FIGS. 5a-5 f are illustrations of cross-sections that reflect structuresthat may result after certain steps are used to make the integratedcircuit of the present invention.

FIGS. 6a-6 h are illustrations of cross-sections that reflect structuresthat may result after certain steps are used to make the integratedcircuit of FIG. 3 following a second method for making the integratedcircuit of the present invention.

FIGS. 7a-7 i are illustrations of cross-sections that reflect structuresthat may result after certain steps are used to make the integratedcircuit of FIG. 4.

FIGS. 8a-8 c are illustrations of cross-sections that reflect structuresthat may result after certain steps used to make the integrated circuitof FIG. 3 are combined with certain steps used to make the integratedcircuit of FIG. 4.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

An improved integrated circuit and method for making it are described.FIG. 3 is an illustration of a cross-section of a section of anembodiment of the integrated circuit of the present invention thatincludes substrate 300 upon which is formed first insulating layer 301.A body strap 302 of a first conductivity type is formed on firstinsulating layer 301. Second insulating layer 303 is formed oninsulating layer 301 adjacent to body strap 302. Film 304 is formed onsecond insulating layer 303 and layer 390. Film 304 extends from surface308 of film 304 to surface 309 of insulating layer 303. Gate electrode305 is formed on top of film 304, and doped regions 306, 307 of a secondconductivity type are formed within film 304. Spacers 312 are shownformed on the sides of gate electrode 305. Spacers 312 space a heavy N+(or P+) implant from gate electrode 305, which may be combined with anearlier N− (or P−) implant to form doped regions 306, 307.

As shown in FIG. 3, doped regions 306, 307 extend from surface 308 offilm 304 to surface 309 of second insulating layer 303. Doped regions306, 307 have junctions 381, 311, respectively. Each junction is spacedfrom body strap 302 by at least about 500 angstroms and preferably atleast about 1,000 angstroms. FIG. 3 shows some degree of misalignmentbetween gate electrode 305 and body strap 302. Some misalignment isinevitable given that gate electrode 305 must be formed on film 304after body strap 302 is formed. Although FIG. 3 shows gate electrode 305shifted to the right with respect to body strap 302, gate electrodecould be shifted to the left instead. Gate electrode 305 is showncovering a significant portion of body strap 302. Although significantoverlap between gate electrode 305 and body strap 302 is preferred, suchoverlap is not necessary for a device to benefit from the advantagesthat the present invention provides. Although not shown, those skilledin the art will appreciate that body strap 302 may be tied to gateelectrode 305 in the conventional manner.

The device shown in FIG. 3 is preferably an SOI structure wheresubstrate 300, layer 390 and film 304 preferably comprise silicon, andinsulating layer 301 preferably comprises an oxide. Other materials,however, may be used to make such structures, as will be apparent tothose skilled in the art. Body strap 302 preferably comprises a lowresistance region, such as one that includes N+ or P+ doped silicon or asilicide. Insulating layer 303 preferably comprises silicon dioxide.Body strap 302, however, could alternatively be formed from otherconductive materials and insulating layer 303 could be formed from adifferent kind of insulating material. In the embodiment of the presentinvention illustrated in FIG. 3, the surface of body strap 302preferably is formed between about 1,500 and about 2,000 angstroms belowsurface 308 of film 304.

The device shown in FIG. 3 may comprise a conventional SOI structure or,alternatively, comprise a structure where insulating layer 303 and layer390, which lies beneath film 304 and contains body strap 302, may bebonded to an oxidized substrate, which comprises insulating layer 301and substrate 300. In the embodiment shown in FIG. 3, film 304preferably is less than about 1,000 angstroms thick, as measured fromsurface 308 of film 304 to surface 309 of insulating layer 303.

Gate electrode 305 may comprise any material that can be used to formsuch a structure, such as doped polysilicon or doped polysilicon cappedwith another conductive material to form a silicide. As shown in FIG. 3,gate electrode 305 overlaps at least part of body strap 302, and thegate length of gate electrode 305 is less than the width of body strap302. Doped regions 306, 307, which may comprise the source and drain forthe device, may be formed by depositing impurities into film 304, thenapplying one or more thermal steps. Doped regions 306, 307 should have aconductivity type opposite to the conductivity type of body strap 302.Where the conductivity type of body strap 302 is “P,” the conductivitytype of doped regions 306, 307 should be “N,” and vice versa.

Junction edges 381 and 311, which correspond to the lower boundaries ofdoped regions 306 and 307, respectively, should be spaced at least about500 angstroms from body strap 302. Such spacing may ensure that thecapacitance produced between junctions 311 and 381 and body strap 302does not significantly slow down the rate at which the body potentialincreases in response to increasing the voltage applied to gateelectrode 305. Such spacing may be created by separating the N+ (or P+)junctions 311, 381 from the P+ (or N+) body strap 302, by placing bodystrap 302 below surface 309 of layer 303, as shown in FIG. 3.

When film 304 is less than about 1,000 angstroms thick, the distancebetween body strap 302 and gate electrode 305 should be at least 50%greater than the distance between surface 309 of layer 303 and surface308 of film 304. For example, when film 304 is about 1,000 angstromsthick, the surface of body strap 302 preferably should be at least about1,500 angstroms from gate electrode 305.

This increased spacing allows body strap 302 to be oversized, i.e, thebody strap may have a width that is greater than the gate length. Suchan oversized body strap should extend into the region beneath channel331, and thus enable the device to benefit from the advantages that adynamic V_(t) provides, even when misalignment between gate electrode305 and body strap 302 is relatively severe. If body strap 302 is notseparated from gate electrode 305 by a sufficient distance, thenmisalignment between the oversized body strap and the gate electrodecould produce a device that suffers from unacceptable capacitancecreated when the body strap is placed too close to the junctions. In theextreme case, a short circuit between body strap 302 and one of thejunctions 311, 381 could result. Such a device is illustrated in FIG. 2.

FIG. 4 is an illustration of a cross-section of a section of a secondembodiment of the integrated circuit of the present invention. Like theembodiment shown in FIG. 3, the device shown in FIG. 4 includessubstrate 400, first insulating layer 401, body strap 402, secondinsulating layer 403, film 404, gate electrode 405, and doped regions406, 407. Like the embodiment shown in FIG. 3, doped regions 406, 407extend from surface 408 of film 404 to surface 409 of second insulatinglayer 403, and junctions 481 and 411 of doped regions 406, 407 arespaced from body strap 402 by at least about 500 angstroms. Body strap402 is tied to gate electrode 405 (not shown).

Unlike the embodiment shown in FIG. 3, film 404 of the device shown inFIG. 4 is greater than about 1,000 angstroms thick. In addition, bodystrap 402 preferably comprises doped polysilicon. In addition, FIG. 4shows regions 413, 414 where additional impurities have been deposited.Depositing impurities into regions 413, 414 serves to extend the N+, N−(or P+, P−) regions that form the upper portion of doped regions 406,407 from surface 408 of film 404 to surface 409 of insulating layer 403.By extending the N+, N− (or P+, P−) regions in this manner, thecapacitance between doped regions 406, 407 and body strap 402 may bereduced from the capacitance that otherwise would have resulted hadthose regions not been extended.

Set forth below is a description of preferred processes for making theintegrated circuits described above, made with reference to FIGS. 5a-5f, FIGS. 6a-6 h, and FIGS. 7a-7 i. Also set forth below is a descriptionof a process, made with reference to FIGS. 8a-8 c, that combines certainsteps from the embodiments disclosed with reference to FIGS. 6a-6 h andFIGS. 7a-7 i to make still another embodiment of the integrated circuitof the present invention.

Those figures illustrate cross-sections that reflect the structuresresulting after performing certain process steps. FIGS. 5a-5 f and 6 a-6h demonstrate alternative methods for making the integrated circuitrepresented by FIG. 3. FIGS. 7a-7 i demonstrate a method for making theintegrated circuit represented by FIG. 4.

To make the integrated circuit represented by FIG. 3, in accordance withthe steps illustrated by FIGS. 5a-5 f, SOI structure 595 may be usedthat includes first layer 590 of semiconductor material that is formedon first insulating layer 501, which in turn is formed on substrate 500.Methods for forming such an SOI structure include separation byimplanted oxygen (SIMOX) or Bonded and Etchback (BESOI)—as is wellunderstood by those skilled in the art. SOI structure 595 preferably hasa first layer 590 that is between about 1,000 and 3,000 angstroms thickand an insulating layer 501 that is less than about 1,500 angstromsthick.

Impurities are deposited into first layer 590, preferably by implantinginto layer 590 ions having a first conductivity type. These depositedimpurities will be used to create the body strap, as described below.For forming an n-MOS device that will include a P+ body strap, boron orindium ions preferably should be implanted. For forming a p-MOS devicethat will include an N+ body strap, arsenic or antimony ions preferablyshould be implanted. Depending upon the structure to be made, ionsshould be implanted at between about 1×10¹⁵ and 1×10⁷ particles/cm³ atan energy sufficient to generate a heavily doped band having a surfacethat is between about 500 and about 1,000 angstroms below surface 598 ofSOI structure 595. A cross-section of the resulting device is shown inFIG. 5a, which shows region 594 that includes the deposited impuritiesformed within layer 590.

Next, hard mask layer 522 is formed on top of surface 598 by applyingconventional techniques for forming such a mask. Mask 522, which may beabout 2,000 angstroms thick and may be made from silicon nitride,silicon dioxide or another suitable material, defines the location wherethe gate electrode will be formed. In this embodiment of the presentinvention, mask 522 is longer than the gate electrode will be. Such anoversized mask helps ensure that part of the subsequently formed channelwill electrically contact the body strap, even if the gate electrode andbody strap are misaligned to some degree. A cross-section of the deviceresulting from the mask formation step is shown in FIG. 5b.

After forming hard mask 522, unmasked portions of layer 590 are removeddown to surface 523 of insulating layer 501. An etching process may beused to produce trenches 524, 525, although other processes may be used.To etch layer 590, chlorine chemistry, such as BCl₃/Cl₂, H₂Cl₂/SiCl₄,ChCl₃/O₂/N₂, or other suitable etch chemistry may be used, as will beapparent to those skilled in the art. After trenches 524, 525 areformed, mask 522 is removed, e.g., by using an etchant that is selectivefor the material used to make hard mask 522 over the material used tomake layer 590. A cross-section of the resulting device is shown in FIG.5c, in which remnants 526 of layer 590 enclose body strap 502.

Next, trenches 524, 525 are filled forming second insulating layer 503adjacent to body strap 502. Preferably, insulating layer 503 comprisessilicon dioxide or silicon oxinitride. When silicon dioxide, thatmaterial may be deposited using any suitable process known in the art,e.g., by using tetraethyl orthosilicate (“TEOS”) in a plasma enhancedchemical vapor deposition (“PECVD”) process. Alternatively, some otherphysical or chemical vapor deposition process may be used to depositsilicon dioxide onto insulating layer 501 to form insulating layer 503.After that deposition step, the surface may be planarized, such as byusing a chemical mechanical polishing step. A cross-section of theresulting structure is shown in FIG. 5d, where layer 503 is preferablybetween about 1,000 and about 3,000 angstroms thick.

In this embodiment of the method for forming the integrated circuitrepresented by FIG. 3, an epitaxial lateral overgrowth (“ELO”) processis used to form an epitaxial layer of silicon 527 on top of remnants 526of layer 590, which also comprise silicon, and on top of insulatinglayer 503 to produce film 504. Epitaxial layer 527 is preferably lessthan about 1,000 angstroms thick. Such an ELO process may be performedusing conventional process steps, but preferably at temperaturessufficiently low to prevent dopants from diffusing from body strap 502into remnants 526. FIG. 5e represents the resulting structure after ELOlayer 527 is polished back.

Conventional process steps may follow for forming gate electrode 505 ontop of gate oxide 530 formed on top of film 504, and for forming dopedregions 506, 507 adjacent to gate electrode 505 and within film 504. Theterm “adjacent,” as used herein, refers to one structure being in closeproximity to another and does not necessarily mean that structures areperfectly aligned or abut one another. For example, although dopedregions 506, 507 are “adjacent” to gate electrode 505, as shown theyextend a relatively short distance underneath gate electrode 505. Dopedregions that extend a relatively small distance below a gate electrodeor spacer, or are spaced a relatively small distance from suchstructures, are considered to be adjacent to such structures as thatterm in used in this specification and in the appended claims.

Gate electrode 505 may be aligned with body strap 502 using conventionalalignment techniques. As shown in FIG. 5f, the gate length for gateelectrode 505 is less than the width of body strap 502. Using such anoversized body strap helps ensure electrical contact between body strap502 and channel 531, even when gate electrode 505 is not perfectlyaligned with body strap 502—as is the case illustrated here. Also, asshown in FIG. 5f, body strap 502 is separated from junctions 581, 511.That separation, which preferably is at least about 500 angstroms, mayensure that the capacitance generated between junctions 581, 511 andbody strap 502 will be lower than would have resulted if the junctionsand body strap had been located closer to each other.

An alternative method for making the integrated circuit represented byFIG. 3 is illustrated in FIGS. 6a-6 h. This alternative technique usesan ioncut/bonded etch-back approach to form a device having a body strapseparated from the junctions. This approach exposes the back side of thedevice to processing before it is inverted to enable the transistor tobe formed on the opposite side.

As shown in FIG. 6a, impurities for creating the body strap aredeposited into a first layer 690 to form doped band 694. Like the methoddescribed above, such impurities preferably are deposited by implantingboron or indium ions, for making a P+ body strap for an n-MOS device, orby implanting arsenic or antimony ions, for making an N+ body strap fora p-MOS device. Alternatively, however, impurities may be deposited onsurface 699 of layer 690, then subjected to thermal treatment, causingthem to diffuse into that layer to form a doped band that extends fromthe surface of layer 690 to a depth of between about 500 angstroms andabout 1,000 angstroms below that surface.

Next, also like the method described above, hard mask 622 may be formedon surface 699 of layer 690, as shown in FIG. 6b. Mask 622 is the gatemask, also shown in this embodiment to be oversized to allow for somedegree of misalignment between the gate and the body strap. After themask formation step, portions of first layer 690 are removed to createtrenches 624, 625 to produce the structure shown in FIG. 6c. Followingthe trench formation step, insulating layer 603 is formed adjacent tobody strap 602 preferably by depositing silicon dioxide over the surfaceof the device, followed by applying a chemical mechanical polishingstep, to produce the structure shown in FIG. 6d.

A metal, such as tungsten, may then be deposited on the surface of thatstructure. That metal may be heated at a temperature that exceeds about700° C. Such a heating step causes the metal to react with the silicon,which contains body strap 602, to create low resistance silicide 686.That reaction takes place over the silicon only, not over oxide 603. Theunreacted metal is removed from the surface of oxide 603 by applying,for example, a conventional wet etch process. That etch step may befollowed by another oxide deposition process, e.g., a PECVD processusing TEOS, then a polished back step to produce the structure shown inFIG. 6e. By adding a silicide to body strap 602, the resulting bodystrap will have a much lower resistance, when compared to a body straplike body strap 502 shown in FIG. 5f which does not include such asilicide.

At this point, and as described in Ser. No. 08/884,921 filed Jun. 30,1997, assigned to this application's assignee, ionized hydrogen isimplanted into layer 690. Hydrogen preferably is implanted at betweenabout 2×10¹⁶-2×10¹⁷ particles/cm³ and at an energy sufficient to producecontinuous stress surface 660 at a distance about 1,000 angstroms belowunderlying surface 662 of insulating layer 603. Following that step,oxidized substrate 691, which includes insulating layer 601 formed onsubstrate 600, is placed on top of layer 690 and insulating layer 603,as shown in FIG. 6f.

This assembly of layer 690, insulating layer 603 and oxidized substrate691 is then heated, preferably to a temperature between about 400°-600°C. until portion 692 of layer 690 delaminates from layer 690 creatingfilm 604. Portion 692 is removed and heating is further continued at atemperature preferably between about 800°-1000° C. until a firm bond isformed between layer 690 and insulating layer 603, and oxidizedsubstrate 691. This assembly, which includes insulating layer 603 andremnants 626 of layer 690 bonded to oxidized substrate 691, is theninverted. A subsequent polishing step applied to the surface of film 604produces the structure shown in FIG. 6g.

Using body strap 602 as an alignment mark, gate electrode 605 is formedon the surface of film 604 to overlap at least part of body strap 602—asshown in FIG. 6h. By using an oversized body strap, contact between bodystrap 602 and channel 631 may be ensured. Subsequently, doped regions606, 607 are formed adjacent gate electrode 605. Doped regions 606, 607extend from surface 608 of film 604 down to surface 609 of insulatinglayer 603. Doped regions 606, 607 are preferably spaced from body strap602 by at least about 500 angstroms. Such separation may ensure lesscapacitance between junctions 681, 611 and body strap 602 results whenan increased voltage is applied to gate electrode 605, than would resulthad the junctions and body strap been located closer together.

FIGS. 7a-7 i illustrate one embodiment of a process that may be used tomake the integrated circuit represented by FIG. 4. This method producesan SOI device having a relatively thick film formed on top of theinsulating layer. The resulting device should, however, ensure that thecapacitance generated between the junctions and body strap, when anincreased voltage is applied to the gate electrode, will be less thanwould have resulted had the junctions been spaced further from the bodystrap. Like the method illustrated in FIGS. 6a-6 h, this method appliesan ion-cut/bonded etch-back approach to form such a device.

As shown in FIG. 7a, polysilicon layer 702 may be formed on the surfaceof first layer 790. Polysilicon layer 702 preferably is between about1,000 angstroms and 2,000 angstroms thick and doped to about2×10¹⁶-2×10¹⁷ particles/cm³ with indium or boron for making a P+bodystrap or with arsenic or antimony for making an N+ body strap. Hard mask763, which preferably comprises an oxide, is formed on top ofpolysilicon layer 702. The resulting structure may be formed usingconventional deposition and etching steps. Although several techniquesfor forming such a structure exist, one such technique deposits apolysilicon layer over first layer 790, dopes that layer, deposits amasking layer over the doped polysilicon layer, then etches the twolayers to form the resulting structure. Hard mask 763 preventsimpurities from being deposited into polysilicon layer 702 during asubsequent doping step. Polysilicon layer 702 will eventually form thebody strap for the device.

Next, spacers 764 are formed on the sides of polysilicon layer 702 andhard mask 763. Spacers 764 are formed to space a subsequent implant fromthe sides of polysilicon layer 702. A preferred method for formingspacers 764 includes the following steps. First, a silicon nitride layeris deposited over the entire surface of the structure. Next, ananisotropic etch step is applied to remove that layer from the device,except where it lies against the sides of polysilicon layer 702 and hardmask 763. The resulting structure is shown in FIG. 7b. By using ananisotropic etch process that selects for silicon nitride over silicon,or whatever other material is used to form layer 790, the siliconnitride may be removed without etching into layer 790.

After spacers 764 are formed, impurities are deposited adjacent to them.Preferably, impurities are deposited by implanting ions into layer790—preferably arsenic, antimony or phosphorus to make an n-MOS deviceor boron or indium to make a p-MOS device. Impurities preferably areimplanted at 1×10¹⁵-1×10¹⁷ particles/cm³ at an energy sufficient to formdoped regions 713, 714, within layer 790 adjacent to spacers 764, thatextend between about 1,000 and about 3,000 angstroms below the surfaceof layer 790, as shown in FIG. 7c. Although ion implantation is apreferred method for depositing impurities into layer 790, solid sourcediffusion or another type of deposition process may be used instead.When making an n-MOS device, polysilicon layer 790 should be doped P+.When making a p-MOS device, polysilicon layer 790 should be doped N+.

After the implant step, silicon dioxide may be deposited over thesurface of the structure, which may or may not retain spacers 764 andhard mask 763. That silicon dioxide layer may then be polished down tothe top of polysilicon layer 702 to produce the structure illustrated inFIG. 7d, which includes insulating layer 703 formed on regions 713, 714formed within layer 790. Using steps like those described above inconnection with FIG. 6e, silicide 786 may be formed on top ofpolysilicon layer 702. Ionized hydrogen may then be implanted at anenergy sufficient to form continuous stress surface 760 preferably atleast about 1,000 angstroms below doped regions 713, 714, as shown inFIG. 7e.

Following steps like those described above in connection with FIG. 6f,oxidized substrate 791 is placed next to body strap 702 (which nowincludes a polysilicon layer capped with a silicide) and insulatinglayer 703 along plane 787, as shown in FIG. 7f. Then, applying heatingsteps like those described above, section 792 of layer 790 is removed,oxidized substrate 791 is firmly bonded to body strap 702 and insulatinglayer 703 and a film resulting from the delamination step is polisheddown to produce film 704, as shown in FIG. 7g.

Subsequently, gate electrode 705 is formed on surface 708 of film 704using body strap 702 as an alignment mark. An N− (or alternatively P−)implant step follows, as shown in FIG. 7h, which may be followed byformation of spacers 712 and an N+ (or alternatively P+) doping step.After applying appropriate thermal steps, junctions 781, 711 are formed,which extend down to surface 709 of insulating layer 703, as shown inFIG. 7i. By extending junctions 781, 711 in this manner, by formingregions 713, 714 prior to forming the subsequently implanted regions,the distance between those junctions and body strap 702 is reduced. Byreducing that distance, the capacitance generated when applying anincreased voltage to gate electrode 705, e.g., when the body tie (notshown) is charging, should be less than would have resulted had thejunctions and body strap remained further apart.

Although FIGS. 7h-7 i show gate electrode 705 and the N−/N+ regionssubstantially perfectly aligned with body strap 702, gate electrode 705and these portions of doped regions 706, 707 may be shifted either leftor right of center. One constraint on the degree of such misalignment isthat the N−/N+ regions should not be shifted so far to either side ofcenter that they cause one of regions 713, 714 to spread beneath one ofspacers 712 to the point where the device's performance is adverselyimpacted.

This process may be varied to produce a device similar to the one shownin FIG. 4, but without having to form spacers 764. As shown in FIG. 8a,such a process begins with a structure like that shown in FIG. 6c, whichmay be made using the steps set forth above for generating such astructure. That structure includes doped band 894 contained within firstlayer 890. Doped band 894 lies between trenches 824, 825. Impurities arethen deposited into first layer 890 to form doped regions 813, 814 belowsurfaces 888, 889 of trenches 824 and 825, as shown in FIG. 8b. Afterthat deposition step, the process steps described above in connectionwith FIGS. 7d-7 i may be applied to create the structure shown in FIG.8c. In this process, the separation between surfaces 888, 889 oftrenches 824, 825 and doped band 894, as shown in FIG. 8b, ensures thatthere will be adequate spacing between body strap 802 and junctions 881and 811, without the need for spacers 764 to provide further lateralspacing.

Features shown in the above figures are not intended to be drawn toscale, nor are they intended to be shown in precise positionalrelationship. Additional process steps that may be used to make theintegrated circuit of the present invention have been omitted when notuseful to describe aspects of the present invention.

Although the foregoing description has specified an integrated circuitthat includes certain features, and has specified certain materials andprocess steps for making such an integrated circuit, those skilled inthe art will appreciate that many modifications and substitutions may bemade. Accordingly, it is intended that all such modifications,alterations, substitutions and additions be considered to fall withinthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for making an integrated circuitcomprising: forming a first insulating layer on a substrate; forming onthe first insulating layer a second insulating layer and a body strap ofa first conductivity type that is adjacent to the second insulatinglayer; forming a semiconducting film on the second insulating layer;forming on the film a gate electrode having a gate length that is lessthan the width of the body strap and that overlaps at least part of thebody strap; and forming a plurality of doped regions adjacent to thegate electrode that extend from the surface of the film to the surfaceof the second insulating layer, the doped regions having junctions, eachjunction spaced from the body strap by at least about 500 angstroms. 2.The method of claim 1 further comprising: forming a doped polysiliconlayer on the surface of a first layer; forming spacers on the sides ofthe doped polysilicon layer; depositing impurities into the first layeradjacent to the sides of the spacers; depositing an insulating layer onthe first layer; removing a portion of the first layer from a firstsurface of the first layer to form the semiconducting film; and bondingan oxidized substrate to a second surface of the first layer.
 3. Themethod of claim 1 further comprising forming a body strap by implantingions into a first layer and applying an epitaxial lateral overgrowthstep to form an epitaxial layer on top of the first layer.
 4. The methodof claim 1 further comprising forming a body strap by implanting ionsinto a first layer, removing a portion of the first layer from a firstsurface of the first layer to form the semiconducting film, and bondingan oxidized substrate to a second surface of the first layer.
 5. Themethod of claim 1 wherein the junctions are each spaced from the bodystrap by at least about 1,000 angstroms.
 6. The method of claim 1wherein the body strap comprises a metal or metal silicide.
 7. A methodfor making an integrated circuit comprising: forming below a firstsurface of a first layer a body strap of a first conductivity type thatis adjacent to an insulating layer; then forming a second surface of thefirst layer that is further removed from the body strap than was thefirst surface; and forming a first doped region that extends from thesecond surface to the surface of the insulating layer.
 8. The method ofclaim 7 wherein the step of forming a second surface comprises applyingan epitaxial lateral overgrowth step to form an epitaxial layer on topof the first surface.
 9. The method of claim 7 wherein the step offorming a second surface comprises removing a portion of the firstlayer.
 10. The method of claim 7 wherein the step of forming a bodystrap that is adjacent to an insulating layer comprises doping the firstlayer below the first surface, masking a portion of the first surface,removing an unmasked portion of the first layer to form a trench, andfilling the trench with an insulating material.
 11. The method of claim7 wherein the body strap is separated from the second surface by betweenabout 1,500 and about 2,000 angstroms.
 12. The method of claim 7 furthercomprising forming a gate electrode on top of the second surface andwherein the body strap is wider than the gate electrode.
 13. The methodof claim 7 further comprising forming a second doped region, the firstand second doped regions each having junctions, each junction separatedfrom the body strap by at least about 500 angstroms.
 14. The method ofclaim 7 wherein the body strap comprises a metal or metal silicide. 15.A method for making an integrated circuit comprising: depositingimpurities into a first layer to form a doped band; etching a trenchinto the first layer; depositing impurities into the first layer belowthe surface of the trench; filling the trench with an insulating layer;removing a portion of the first layer from a first surface of the firstlayer to form a semiconducting film; and bonding an oxidized substrateto a second surface of the first layer.